1. Field of the Invention
The present invention relates to a structure of a magnetic field measuring apparatus, and more particularly relates to a structure of a gas cell of an optically pumped magnetometer, and a method for manufacturing the apparatus.
2. Description of the Related Art
A high-sensitivity optically pumped magnetometer highly integrated into a small size has been developed, and there is W. Clark Griffith, Svenja Knappe, and John Kitching, “Femtotesla atomic magnetometry in a microfabricated vapor cell”, Optics Express 18, 27167-27172 (2010) as background art in this technical field.
A gas cell of an optically pumped magnetometer described in W. Clark Griffith, Svenja Knappe, and John Kitching, “Femtotesla atomic magnetometry in a microfabricated vapor cell”, Optics Express 18, 27167-27172 (2010) is configured such that both surfaces of a silicon substrate in which a penetrating hole (cavity) is formed are bonded with glass substrates, and is made of a three-layer structure including glass/silicon/glass to seal rubidium chloride (RbCl) generating alkali metal gas atoms in the penetrating hole and barium azide (BaN6) generating nitrogen gas serving as buffer gas.
The magnetometer using the gas cell operates as follows. Circular-polarized pump light is emitted to the alkali metal gas atoms sealed in the gas cell, whereby the alkali metal gas atoms are spin-polarized by optical pumping method. Subsequently, linearly-polarized probe light is emitted in a direction perpendicular to the pump light, the plane of polarization of the probe light is rotated by magneto-optic effect called Faraday rotation. The plane of polarized light rotates by an angle proportional to the static magnetic field strength in a direction perpendicular to the optical path of the probe light, and therefore, the magnetic field strength can be measured by causing a light detection device to detect the angle of the plane of polarization of the probe light that has passed the gas cell.
A magnetometer that can be achieved using a similar gas cell is a magnetometer called a magneto-optical double resonance-type optically pumped magnetometer, or an Mx-type optically pumped magnetometer. The magneto-optical double resonance-type optically pumped magnetometer operates as follows. Circular-polarized laser light is emitted to the alkali metal gas atoms sealed in the gas cell, whereby the alkali metal gas atoms are spin-polarized by optical pumping method. The spin-polarized alkali metal gas atoms make precession movement with a frequency proportional to the static magnetic field strength in the gas cell. At this occasion, when an RF magnetic field of the same frequency as the frequency of the precession movement is applied to the gas cell, the light that passes the gas cell indicates the resonance peak due to the magneto-optical double resonance. This resonance peak is detected by the light detection device, and the static magnetic field strength can be measured from conversion from the frequency of the applied RF magnetic field (corresponding to the frequency of the precession movement).
As described above, in any of the magnetic field measuring methods, the optically pumped magnetometer performs the magnetic field measuring by using the spin-polarization state of the alkali metal gas and the change in the optical information generated in accordance thereto. Therefore, in the optically pumped magnetometer, it is important to maintain the spin-polarized state of the alkali metal gas atoms for a long time. In order to solve such problem, W. Clark Griffith, Svenja Knappe, and John Kitching, “Femtotesla atomic magnetometry in a microfabricated vapor cell”, Optics Express 18, 27167-27172 (2010) describes a method for sealing not only the alkali metal gas atoms but also rare gas and nonmagnetic gas such as nitrogen gas (which are called buffer gases) in the gas cell. The time for which the alkali metal gas atoms are in the spin-polarized state depends on the pressure of the sealed buffer gas.
In the optically pumped magnetometer, the magnetic field measuring is performed by detecting the light that passes through the gas cell. For this reason, in order to prevent degradation of the light transmittance in the area where the light passes and to prevent reduction of the detected signal level, alkali metal solid matter such as rubidium and chemical compounds sealed in the gas cell such as barium azide need to be prevented from attaching to the area. In order to solve such problem, W. Clark Griffith, Svenja Knappe, and John Kitching, “Femtotesla atomic magnetometry in a microfabricated vapor cell”, Optics Express 18, 27167-27172 (2010) describes a structure in which two penetrating holes having a dimension of 3 mm×2 mm×1 mm and a narrow passage having a dimension of 1 mm×0.1 mm×1 mm connecting the penetrating holes are formed in the gas cell, and the rubidium chloride and the barium azide are sealed in one of the penetrating holes, whereby unreacted residue and alkali metal are less likely to diffuse to the other of the penetrating holes where the light for the magnetic field measuring is passed.